Highlighting text too slow (Sigil Qt5)

Hello folks,
I have a problem with Sigil 0.8.4-1 (it uses Qt5) on ArchLinux 32bit:
https://www.archlinux.org/packages/community/i686/sigil/

Highlighting text (in Code View) is very slow. That happens only if I copied some text from other programms.
I made a video who shows this strange bug:
http://quellen.altervista.org/Sigil_qt_bug.mkv

The developer of Sigil says that maybe it's a Qt bug:
https://github.com/user-none/Sigil/issues/39#issuecomment-73254809